1. Field of the Invention
This invention relates in general to equipment cleaning and more particularly to a method and composite article for cleaning synthetic resins stains and residue out of the molds used in molding usable products from such materials.
2. Description of the Prior Art
The uses of compression molds, transfer molds or any other type of molds having a platen for forming useful products out of thermohardening compounds, such as epoxies, is a comparatively old and well developed technology. As is the case with most equipment, molds must be periodically cleaned and this is a tedious and time consuming task due to the complex configurations of many of todays molds. For a complete understanding of the problems associated with mold cleaning, a discussion will now be presented of a particular type of mold commonly used in forming semi-conductor products.
In the manufacture of integrated circuit packages, a plurality of the discreet integrated circuit chips are bonded in an aligned spaced side-by-side relationship on a thin metallic strip commonly referred to as a "leadframe". Such leadframes, usually two, are placed in spaced parallel positions on the bottom mold of a heated transfer-type mold set and a top mold of the set is moved downwardly into a precisely registered position atop the bottom mold and the two molds are clamped together. The bottom mold is provided with a centrally aligned row of material receiving cavities each of which has a plurality of channels, which are referred to as "runners", extending therefrom into communication with a similar number of upwardly opening recesses of the bottom mold. The top mold is formed with downwardly opening recesses which cooperate with the recesses of the bottom mold so as to enclose each of the discreet integrate circuit chips in its own chamber. The top mold is formed with plural vertical bores therein each of which aligns with a different one of the centrally located material receiving cavities of the bottom mold and a plunger is demountably and slidably mounted in each of the bores.
To accomplish a molding operation, the plungers are removed from the vertical bores of the top mold and the epoxy resin material, usually in pellet form, is dropped down through the bores into the material receiving cavities of the bottom mold. The plungers are reinserted into the bore of the top mold and a downwardly directed force is exerted thereon. The pressure exerted by the plungers along with heat from the molds will turn the epoxy resin pellets into a molten state causing the molten material to flow through the runners into the chambers of the mold. When this occurs, the molten resin starts to cure and a short time later, each of the discreet integrated circuit chips will be encapsulated in a solidified package which is shaped in accordance with the configuration of the chamber in which it was located prior to commencement of the encapsulating process.
When the curing step is completed, the bottom and top molds are separated and ejector pin mechanisms provided in both of the molds are operated to eject the leadframes from the molds. The leadframes are then subjected to further processing including cull removal, package separation from the leadframes trimming and the like.
As in all production operations, the above product encapsulation steps are repeatedly accomplished and, as hereinbefore mentioned, the molds must be cleaned periodically, sometimes as often as two or three times in an eight hour shift.
After each molding operation, or "shot" as it is commonly referred to in the art, some epoxy residue is left behind in the molds and produces staining of the molds. If the molds are not cleaned at proper intervals, mold staining causes an unacceptable finish on the molded products. The unacceptable finish typically appears in the form of cloudy spots on the finished products, that is, on the exterior surfaces of the integrated circuit packages of the above presented example. In addition to the need for cleaning the molds to remove stains, in some instances, flashing residues will build up on the parting surfaces of the two mold halves due to inaccuracies of the molds per se, varying thicknesses of the leadframes, and the like.
In the molding industries in general, and in the semiconductor arts in particular, there are basically four prior art mold cleaning techniques currently being used. Three of these prior art techniques utilize especially formulated melamine molding compounds which have an inherent affinity for the epoxy residues left behind in molding operations and thus are used to clean the molds.
In a first one of these prior art mold cleaning techniques, a compression grade of melamine molding compound in powder form is sprinkled on the bottom mold so that the parting surface of the mold is covered with as even a layer as is possible. The mold is then closed, e.g. the top mold is placed atop the bottom mold, and they are clamped together with a force which is considerably less than that applied during a normal production use. The molds, which remain heated during the cleaning operation, along with the compressive forces will cause the powdered melamine compound to melt and flow into conformity with the interior configuration of the mold set and subsequently cure. When the curing operation is completed, the molds are opened and the ejector pin system is operated to eject the molded layer from the parting surfaces, cavities, runners, and other recessed surfaces of the molds. However, there is no guarantee that the molded layer will be ejected in a single piece and, in fact single piece ejection very seldom occurs. This is due to spillage of the powered melamine, prior to molding, into the multiple holes and the various recessed portions of the mold surfaces and due the ejector pins not being provided in all of the holes, cavities and other recessed surfaces of the molds. Therefore, the cured melamine layer will break into pieces with many pieces remaining in the mold. Very often, the remaining pieces of the cured melamine will need to be chiseled out by a special tool made of soft material so as not to damage the molds. In addition, mold damage can result from excessive amounts and/or uneven distribution of the powdered melamine at the beginning of the cleaning operation, and the subsequent application of clamping forces. In such instances, the molds can shift which results in mis-alignment and possible mold breakage. Another problem with this first mold cleaning technique is that when the powdered melamine is sprinkled onto the hot base mold it will immediately begin to melt and thus start to cure before the molds set is closed and clamped together. This causes a reduction in the mold cleaning capabilities of the melamine compound. In order to minimize this premature melting and curing problem, sometimes the mold set is intentionally allowed to cool down somewhat from its normal production temperature. This, while not completely eliminating the premature melting and curing problem, will minimize the problem, but the lowered mold temperature significantly increases the cure time of the melamine and thus slows down the cleaning operation.
The above described mold cleaning operation is normally repeated several times, usually between three and six times, until the mold set is completely cleaned. In that the cured melamine usually breaks into a plurality of pieces some of which need to be chiseled out of the mold, it is difficult, if not impossible, to inspect the cured melamine for determining when the mold cleaning operation is completed. Therefore, it is necessary that the mold set itself be visually inspected, as opposed to inspecting the cured meelamine and this is difficult due to the multiplicity of cavities, runner and other surfaces of the mold. Also, due to spillage of the powdered melamine, and breakage of the cured melamine, inspection of the cured melamine, rather than inspection of the molds themselves, for proper ejector pin retraction is impossible.
The second mold cleaning technique sometimes used is commonly referred to as the "preform" method. In this technique, the melamine is compressed or otherwise formed into solids which are placed on the bottom mold surfaces. The heat and the compressive forces applied by the mold set will melt the preforms and cause the molten melamine to spread out on the parting surfaces of the molds ad flow into the cavities, runners and other recessed surfaces of the molds. The use of preforms provides an improved even coating of the parting surfaces of the molds in comparison to the above described powdered melamine technique. However, breakage of the cured melamine upon removal from the mold, the need for chiseling, visual inspection, and the other control problems associated with the aforesaid first technique are about the same for this second, or preform, method.
A third prior art mold cleaning technique is sometimes used and this technique utilizes a different grade of melamine molding compound of the type suitable for transfer molding operations. The transfer melamine molding compound is placed in the mold set in exactly the same way that the epoxy molding compound is placed therein and the cleaning procedure is exactly the same as an epoxy molding, or production, operation. This transfer type mold cleaning technique is not nearly as efficient in so far as its mold cleaning capabilities as the hereinbefore described compression cleaning operations. In that the transfer mold cleaning technique is accomplished with full clamping forces being applied to the mold set, the transfer grade melamine will not reach the parting surfaces of the mold set and therefore cannot clean those surfaces. In that the mold cleaning technique is used in a normal transfer molding manner, it must be accomplished with dummy leadframes in the molds. Also, the transfer grade melamine molding compound has a viscosity which often blocks, or otherwise interferes, with moving mold components such as ejector pins, floating cavities and the like. Also, the transfer melamine molding compound cleaning technique needs to be combined with some other cleaning procedure in order to clean the parting surfaces of the mold halves.
The fourth prior art method for cleaning molds is accomplished by cooking the contaminated parts of a disassembled mold set in a suitable detergent such as N-Pyrol. This technique is, of course, unacceptable except possibly at the end of a production run, in that it takes the mold out of service for an excessively long period of time.
Therefore, a need exists for a new and improved article and method for cleaning molds which overcomes some of the problems and shortcommings of the prior art.